The present invention relates to the selection of a limit value, such as an alarm limit, from a range of limit values, in particular for patient monitoring systems.
In today's patient monitoring systems, one of the most important tasks is to monitor the condition and/or status of a patient and to alarm medical staff in case that one or more monitored physiological parameters of the patient exceed a predefined upper or lower alarm limit. These alarm limits may either be set manually, e.g. by the medical staff, or can be set automatically, e.g. at a start of a measurement or on a user request. The alarm limit may be set as a fixed limit for each physiological parameter or can be based upon one or more current values of the physiological parameter of the patient e.g. determined by the patient monitoring system.
U.S. Pat. No. 4,994,790 discloses a method for setting an alarm in an apparatus for monitoring a plurality of patients. A present time value, an upper and a lower limit of a living body signal of one of the patients is indicated. A limit or threshold value of the upper limit and the lower limit can be set based upon the present time value of the living body signal by depressing an appropriate key of a keyboard.
U.S. Pat. No. 5,226,416 discloses an apparatus for monitoring output signals from a sensor. Upper and lower alarm levels are defined based on initial value or values determined by the sensor.
In case that the alarm limits are set automatically by the monitoring device, there are several ways to calculate the alarm limit based upon the patient's signals. In U.S. Pat. No. 5,226,416 and U.S. Pat. No. 4,994,790, a linear formula is used to calculate the upper and lower alarm limits. In general, simple formulas are used to calculate the alarm limits either by adding/subtracting a parameter specific offset to/from a current value, by multiplying the current value with a parameter specific factor, or by a combination of both. However, all of the known algorithms to calculate the alarm limits exhibit the disadvantage that they are not flexible enough and might lead to an unnecessary alarming of the monitoring system.